Without some means of power factor correction any mains connected equipment which rectifies the incoming AC supply to produce a DC supply will be characterised by low power factor and high harmonic distortion which will generally exceed the permitted limits for mains-connected equipment. Power Supply Units (PSUs) and lighting ballasts which are designed for high efficiency, cost-sensitive consumer applications are often switched-mode types and are frequently based upon the half-bridge or full-bridge topology. These topologies are particularly suitable for higher power, high efficiency applications in which the ratio of input to output voltage is relatively confined. Regulations have been introduced in recent years to constrain the way that the input current is drawn from the AC supply, including the Power Factor (PF), Crest Factor (CF) and Total Harmonic Distortion (THD). The continuing pressure to conform to tighter regulations and reduce manufacturing costs forces the need for innovative methods in the design of switched-mode power supply controllers.
Various passively switched Power Factor Correction (PFC) circuits have been invented which use the switching power waveforms of the power converter to provide a measure of PFC to enable products to meet the statutory regulations at low cost with the disadvantage that the output current through the output load has a high ripple content. However, in many applications it is desirable that the current through the output load is substantially constant with low ripple. For example, in the case of LED lighting, a constant output current with low ripple provides advantages of high efficiency and long life as well as high quality light output without flicker.
Such prior circuits include those disclosed in U.S. Pat. No. 7,911,463B2, US20090251065A1, WO2008152565A2, WO2010054454A2, WO2010143944A1, and WO9204808A1. Although these prior circuits achieve high PF with respect to the way that power is drawn from the mains supply, these circuits are generally not capable of delivering a current to the load which is both regulated and has low ripple.